The Samuel Roberts Noble Foundation, Inc.
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Are Low Rate Herbicide Applications Worth the Risk?

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With the tight profit margins agricultural producers face, the need to control input costs is greater than ever. As input costs rise, we naturally try to reduce these costs whenever we can. One cost-cutting approach producers often ask about is reducing herbicide rates, sometimes to below the minimum listed on the label. It is an option, but consider these pros and cons before taking this approach to cutting costs.

Pros:

  • There is the potential for saving up to 50 percent of the herbicide cost per acre depending on the species of weeds present and their herbicide susceptibility. For low rates to be effective, the weeds must be a highly susceptible species and be treated when they are very small.
  • Low rates may reduce the potential for adverse environmental impacts.

 

Cons:

  • Because low rate applications must be made early when the weeds are small, the potential for later weed flushes that would require a second application increases.
  • Careful scouting is required to identify the weed species and estimate their density before they get too large. Producers must invest the time to get out and walk their fields and pastures to see the small weeds.
  • The application window is very short to treat the weeds while they are small and susceptible. Any delays caused by wind, rain or other conflicts can allow the weeds to get too large to be controlled by a low rate.
  • Small weeds are more susceptible to herbicides, but since the target crop is smaller, it may also be more susceptible to herbicide injury.
  • The cost savings are only for the herbicide. The application cost remains the same regardless of herbicide rate.
  • The potential for a control failure is higher with below-label rates. Chemical manufacturers conduct tests over a wide range of environments and select rates they believe will provide the most consistent performance. If a below-labeled rate is used and a failure occurs, the producer assumes all of the liability.
  • In cases where a failure occurs, the surviving weeds can go to seed and recharge the seedbank to become future weed problems.
  • Only highly susceptible weed species will be controlled at low rates. Less susceptible species listed on the label may not be controlled and will continue to pose problems.

 

An additional concern is the potential development of herbicide resistance. In some cases, the use of low herbicide rates has selected for individual weeds within a species with some herbicide resistance. Those surviving plants then reproduce and their resulting progeny have an increased level of resistance. In one experiment using several rates of a known herbicide on susceptible ryegrass, there were plants that survived the low rate application. These plants were then allowed to reproduce, and, within four generations, a completely herbicide-resistant population developed. If you suspect herbicide resistance, switch to a herbicide with a completely different mode of action that is labeled for the weed and use the full, labeled rate.

Another issue is the legality of using below-label rates. The rates listed on herbicide labels are approved by the EPA and any application above those rates is illegal. However, some state officials say that a user can legally choose a rate lower than listed on the label unless the label specifically prohibits lower rates. It is best to check with your state department of agriculture to find out if they consider below-label rate applications a violation.

Use of low rates is an option to consider in certain situations, but it carries significant risks that the producer must be willing to accept. The producer must decide if the potential cost savings is worth accepting the associated risks.